Previous simplified optimal life history models suggest that natural selection favors all-or-none energy allocation between somatic growth and reproduction such that juveniles grow in body size but do not reproduce while adults reproduce but do not grow. This ideal pattern departs greatly from energy allocation by the organisms that comprise the majority of living biomass on earth, as these both grow and reproduce more or less simultaneously during most of the life cycle. This paper explores the possibility that this commonly observed energy allocation pattern arises from density dependence in population growth, a feature of nature not addressed by the simplified models.
Assumptions concerning vital rates of individual organisms give rise to a very general continuous time model of structured population growth. From this model emerges a criterion that characterizes the energy allocation pattern most favored by natural selection in a limited, constant environment. Exploration of this criterion reveals that density dependence fails to alter the optimality of the ideal all-or-none energy allocation pattern of the simplified models. This result strengthens the suspicion created by several recent theoretical investigations that simultaneous growth and reproduction represents an evolutionary response to environmental variability.